Study On The Process And Properties Of Semi-solid Powder Compression Molding Of Medical Mg-Zn Allo

Semi solid powder molding is a new and efficient powder metallurgy technology.It has a series of advantages,such as short process,low energy consumption,near net-shape forming,etc.a new kind of powder metallurgy process,named semi-solid powder molding,was adopted in this study.The mechanical properties and biological corrosion resistance of the alloy were improved by adding Zn element alloying.The medical Mg-Zn based alloys with different Zn contents were prepared to achieve the purpose with high performance and corrosion resistance.Scanning electron microscope(SEM),energy dispersive spectrometer(EDS),X-ray diffractometer(XRD),Hot compressor,universal compression tester were used to analyze the microstructure,mechanical properties of the samples.The process of forming process of semi-solid powder molding was explored.Finally,electrochemical experiments and simulated liquid dynamic immersion experiments were used to analyze the biological corrosion performance and corrosion mechanism under different parameters and alloy composition.The results show that the relative density and microhardness of the alloy increase with the increase of temperature and holding time.In addition,the compressive strength of Mg-3Zn alloy prepared at 600℃ can reach more than 300 MPa,but when the temperature rises to 620℃,the compressive strength drops to 191 MPa.Therefore,too high temperature will reduce the mechanical properties.When the holding temperature is 600℃,the relative density and microhardness of mg-9zn alloy reach the maximum value with the holding time extended to 20min,which are 97.9%and 115.8hv respectively.The results of XRD,SEM and EDS show that the microstructure of the alloy is mainly composed of α-Mg matrix and the second phase(MgZn2,Mg4Zn7 and Mg2Zn11).The grain morphology is mostly equiaxed and nearly spherical,with the grain size of about 30-40 μm.With the increase of Zn,the grain size gradually decreases.Therefore,in the process of semi-solid powder forming,deformation,solidification and densification occurs simultaneously.With the increase of forming temperature,holding time and Zn content,not only the liquid content increases,but also the flow of the liquid phase is accelerated,making the liquid phase filling more fully,and finally the relative density and hardness increase.The compression experiment of as cast Mg-6Zn alloy was carried out in semi-solid state,and then it was modified.The semi-solid compression can be divided into three stages:quasi elastic deformation,strain hardening deformation and rheological viscoplastic deformation.They participate in the whole process of semi-solid compression of magnesium alloy.The main reasons for the compression deformation of semi-solid are the sliding or rotation of solid particles along the liquid film,the sliding between solid particles,the plastic deformation and hardening behavior of solid particles,whose effect depends on the deformation parameters.The Tafel curves of Mg-Zn alloys prepared under different parameters were analyzed.The dynamic immersion experiments were carried out in Hank’s liquid constant temperature oscillator at 37℃ for 3,6,9 days.The biological corrosion performance and biodegradation behavior of Mg-xZn(x=3,6,9)alloy prepared by semi-solid powder were studied.The results show that both the relative density and the second phase affect the corrosion performance.The corrosion rate decreases with the increase of temperature,which is due to the increase of relative density.With the increase of zinc content,the corrosion rate firstly decreases and then increases,which is due to the increase of the number of the second phase and the uneven microstrcture.Finally,the corrosion rate of Mg-6Zn alloy is the lowest,and the corrosion rate of Mg-6Zn alloy prepared at 600℃ is 4.8mm/year after 9 days of corrosion.Through the analysis of pH change and weight loss,the corrosion mechanism is as follows:the primary corrosion process is dominated by the second phase,and the later corrosion process is mainly affected by the relative density.The degradation mechanism of Mg-Zn alloy in Hank’s solution,and XRD analysis results show that the corrosion products are mainly composed of Mg(OH)2,hydroxyapatite(HA),a small amount of MgO and CaCO3.